The primary purpose of reservoir characterization is to gain a complete understanding of petrophysical properties. The objective of this essay is to understand laboratory measurements and basic definitions of petrophysical properties: porosity, permeability, relative permeability, capillarity, and saturation. Pore ​​size distribution is presented as the common link between these properties. Rock and fluid properties are the building blocks of any reservoir engineering study that lead to the formulation of a successful reservoir management strategy. Sometimes the study involves estimating oil and gas reserves based on a simple analytical approach, as demonstrated in this chapter. On a separate note, performance prediction of oil and gas reservoirs is done using multi-dimensional simulation models and robust multi-phase systems. Regardless of the study and its complexity, the reservoir engineer must have a thorough understanding of the properties of the rock involved. What is more important is knowledge of the variability of rock properties throughout the reservoir and the behavior of heterogeneous reservoirs in the real world. It is a common observation that rock properties vary from one location in the reservoir to another, often impacting reservoir performance. Some reservoir analyzes are based on the assumption that a reservoir is homogeneous and isotropic, which implies that the rock properties are nonvariable and uniform in all directions. In reality these conditions are so idealized that they are rarely met in the field. Various geological and geochemical processes leave imprints on a reservoir over millions of years, leading to the occurrence of reservoir heterogeneities that were largely unknown before oil and gas production. For example, the occurrence…half of paper…n can be derived from empirical relationships with other measurements such as porosity, NMR, and sonic recording. The thickness of the rock will have sufficient permeability to transcend fluids to a WELL hole. This feature is usually called “net reservoir rock”. In the oil and gas industry, another “net pay” quantity is calculated, which is the thickness of rock that can transcend the hydrocarbons in the well at an optimal rate. Reservoir models are constructed from their measured and derived properties to estimate the amount of hydrocarbons present in the reservoir, the rate at which the hydrocarbon can be produced at the Earth's surface through wells, and the fluid flow in the rocks. For the water resources industry, models are similarly used to analyze the amount of water that can be produced at the surface for long periods of time, without depleting the aquifer.